At present, as a composing material of absorbing goods such as disposable diapers, sanitary napkins, incontinence pads, a water-absorbent agent containing a water-absorbent resin as a main component, in addition to a hydrophilic fiber such as pulp, is widely used.
Such a water-absorbent resin is a water-insoluble and water-swellable cross-linked polymer, having two fundamental properties, that is, to absorb a large amount of water, and not soluble in water (amount of water-soluble component is small). In recent years, with high functionalization of absorbent goods such as disposable diapers, many improvements have been proposed. In addition, to respond to requirement for possibility to use absorbent goods over a long time (for example 16 hours at nighttime), or for making thinner, there is such tendency that concentration of a water-absorbent resin in a water-absorbent agent is increased (mass ratio of a water-absorbent resin is increased), thus importance of property of a water-absorbent resin is increasingly strengthening.
That is, as a fundamental characteristics required to a water-absorbent agent (a water-absorbent resin), it was conventionally required that amount of water-soluble component is small, and absorption capacity is high in contacting with aqueous liquid such as body fluid. However, in recent years, absorption rate, fluid permeability, strength of swollen gel, and amount of sucking to suck water from a substrate containing aqueous liquid and the like have been further required as absorbent goods. In addition, it has been similarly required that particle size distribution is very narrow, and not only absorption capacity without load but also absorption capacity under load and fluid permeability under load are high.
For example, with higher functionalization of absorbent goods, many parameter patens specifying various properties of the water-absorbent agent (water-absorbent resin) have been proposed as the following patent literature.
For example, in reissue U.S. Pat. No. 32,649, there are disclosed a water-absorbent resin excellent in absorption capacity, gel strength and water-soluble component, and an invention of a hydrogel-forming polymer composition having an amount of water-soluble component in synthetic urine after 1 hour of equal to or lower than 7.5% by mass, and an amount of water-soluble component in an equilibrium state of equal to or lower than 17% by mass. In U.S. Pat. No. 6,087,002, there is disclosed a water-absorbent resin with controlled particle size, having an amount of dissolution soluble components of equal to or lower than 1.68% by mass. In U.S. Pat. No. 5,985,944, there is disclosed a water-absorbent resin excellent in absorption capacity under load, residual monomers, water-soluble component and pore diameter of foaming. In U.S. Pat. No. 5,601,542, there is disclosed a water-absorbent resin excellent in pressure absorption capacity index (PAI) and an amount of water-soluble component of equal to or lower than 16% by mass. In U.S. Pat. No. 6,127,454, there is disclosed an invention of a water-absorbent resin having a water-soluble component for 1 minute of equal to or lower than 1% by mass, high absorption capacity under load and high absorption efficiency in an upper and lower gel layer. In U.S. Pat. No. 6,602,950, there is disclosed an invention of a water-absorbent resin excellent in absorption capacity and vertical absorption capacity under load, having a soluble component of equal to or lower than 4% by mass. In U.S. Pat. No. 6,060,557, there is disclosed a water-absorbent resin excellent in absorption capacity without load, absorption capacity under load, swelling pressure after 20 minutes, water-soluble component (from 3.5 to 10%) and maximal re-wetting amount. In U.S. Pat. No. 5,797,893, there is disclosed an invention of an absorbent agent composition having a diffusion absorption index of equal to or higher than 1.5 g/g/min., which represents maximal absorption amount per unit hour, wherein mass of a normal saline solution absorbed by the absorbent agent composition over 60 minutes is determined with time. In U.S. Pat. No. 5,760,080, there is disclosed an invention of a water-absorbent agent having a diffusion absorption capacity at 60 minutes after starting absorption, of equal to or higher than 25 g/g/min., and an amount of water-soluble component of over 0 and equal to or lower than 7% by mass. In U.S. Pat. No. 4,666,975, there is disclosed a water-absorbent resin excellent in three properties, absorption capacity, gel strength, water-soluble component, and water-absorption rate. In U.S. Pat. No. 6,187,872, there is disclosed a water-absorbent resin with low water-soluble component, which is an absorption capacity under load of equal to or higher than 20 g/g, obtained by polymerization of non-neutralized acrylic acid and then by neutralization thereof. In ERT 470. 1-99 (published in 1999), there is disclosed a measurement method for water-soluble component of a water-absorbent resin (ERT; EDNA Recommended Test), standardized by EDNA (European Disposables and Nonwovens Association), and found values of from 5.29 to 9.00% on samples A to C are described.
It should be noted that in these patent literature or non-patent literature, amount of water-soluble component is quantitatively determined by stirring water-absorbent resin particles in a large excess (from several-hundred times to several-thousand times) of water or a normal saline solution or artificial urine, extracting a water-soluble polymer from water-dispersed swollen gel particles under stirring, and by titration thereof.
It should be noted that a water-absorbent resin, which composes a water-absorbent agent which has been produced industrially in a large quantity at present, is generally produced by subjecting particles of an internally cross-linked partially neutralized polyacrylic acid-type polymer (water-absorbent resin precursor) to heat treatment in the presence of various surface cross-linking agents to introduce a cross-linked structure at the vicinity of the surface of said particles. By such surface cross-linking treatment, the water-absorbent resin precursor becomes water-insoluble and water-swelling, and is capable of exerting water-absorbing performance. In order to provide water-absorbing performance to the partially neutralized polyacrylic acid-type polymer (water-absorbent resin precursor) in this way, surface cross-linking treatment may be carried out once to the polymer. However, also technology to carry out said surface cross-linking treatment twice has been proposed.
For example, in Example 5 of the specification of the above U.S. Pat. No. 5,797,893, there is disclosed that a water-absorbent resin was obtained by subjecting a water-absorbent resin precursor to surface cross-linking treatment twice, by using a solution of a surface cross-linking agent composed of glycerine/ethylene glycol diglycidyl ether/water/ethyl alcohol. It should be noted that, in said Example, a composition of the solution of the surface cross-linking agent is the same in two times of the surface cross-linking treatment. In addition, content of water in the treatment liquid relative to mass of the polymer to be subjected to surface cross-linking treatment, is the same in two times.
Also in the pamphlet of WO 95/09014, there is disclosed technology to carry out the surface cross-linking treatment twice. Specifically, in Examples 1 to 3 of said pamphlet, the second time surface cross-linking treatment is carried out by mixing a treatment liquid containing a surface cross-linking agent to a comparative polymer and heating. It should be noted that in Example 1 of said pamphlet, content of water in the treatment liquid (relative to the polymer) is the same in the first time and the second time of the surface cross-linking treatment. In addition, in Examples 2 and 3, content of water in the treatment liquid (relative to the polymer) used in the second time is two times content of water in the treatment liquid used in the first time of the surface cross-linking treatment.
Also in the pamphlet of WO 2006/123561, there is disclosed technology to carryout surface cross-linking treatment twice (for example, claim 1). By referring to Example of said pamphlet, a precursor of water-absorbent resin particles before surface cross-linking is produced by reversed phase suspension polymerization, and content of water (moisture regain) in the precursor of the water-absorbent resin particles in the surface cross-linking treatment twice is relatively high, about 40% and about 30%, respectively, and thus it is considered that the surface cross-linking treatment is carried out always in a gel-like state.